Fix LightRevealScrimRepositoryTest on Robolectric.

The test was failing with "no instrumentation registered". Per the
accepted answer of this problem
(https://yaqs.corp.google.com/eng/q/8977167194139918336), we could use
`AndroidJUnit4` test runner.
Before this fix, we got http://ab/I36700010187829959.
With this CL, we got http://ab/I27200010186873167.

Test: atest -c SystemUiRoboTests (http://ab/I27200010186873167)
Change-Id: Iefc9c89c1bce5982b823dcc88ea450e341d09532
Merged-In: Iefc9c89c1bce5982b823dcc88ea450e341d09532